The agent of Borussia Dortmund contract rebel Robert Lewandowski has revealed Manchester United have expressed an interest in signing the striker.

United manager Sir Alex Ferguson and his assistant Mike Phelan were both in Germany on Saturday to watch Borussia Dortmund beat Bayern Munich 5-2 in the German Cup final, with Japanese playmaker Shinji Kagawa believed to be the primary focus of their attentions.

The Premier League runners-up have also been linked to Dortmund centre-back Mats Hummels in the past, but Lewandowski's agent, Maik Barthel, has now revealed the English club have expressed an interest in signing the Pole.

With Dimitar Berbatov tipped to leave Old Trafford in the summer United will need to add to their striking options - with Lewandowski's uncertain contract situation making the 23-year-old especially attractive.

"I can confirm that the [new contract] offer of Borussia Dortmund is insufficient," Barthel told Bild. "I am also able to confirm an interest of Manchester United in the services of Robert Lewandowski."

Dortmund are adamant they are not prepared to sell their forward, however, after he scored 22 goals this season to help the club claim the Bundesliga and German Cup Double.

His current contract expires in the summer of 2014, but Dortmund managing director Hans-Joachim Watzke has insisted neither he nor Hummels will be leaving in the near future.

"We expect Kagawa to go, but no amount of money will persuade us to sell the others," Watzke said.